Elemental metal materials, especially those offering a high purity of elemental metal, are processed into various shapes and sizes according to the required applications, such as powder-molded products, sheet metals, bars, thin wires and foil materials.
In recent years, metal powder offering high purity is drawing the attention as an effective molding material for use in various molding processes such as powder metallurgy and thermal spraying. Powder metallurgy is an important technology used in wide-ranging fields including the production of mechanical parts. Accordingly, demand for metal powder as a powder metallurgy material is also growing.
The traditional methods for producing metal powder include the classic method of mechanically and directly crushing metal particles into powder and the method to blow molten metal using gas and shape the blown droplets into powder form. However, these methods posed problems such as irregularities in particle shape and size, poor economy, and so on.
Electrolysis is one of the relatively new methods known for producing metal powder. However, electrolysis has been reported to produce metal of a brittle spongy or powder structure if metal deposition is implemented under a condition outside the optimal range where metal of a smooth, fine and uniform crystalline structure can be deposited. Even when deposition is achieved under an optimal condition, the metal powder obtained by the electrolysis method as presently known does not satisfy the required levels of purity or uniformity of metal particle shape and size. Other problems associated with this methods, such as poor economy, also remain unresolved.
Among other metals, titanium is a relatively new metal compared with iron or copper that has been known since the ancient times or aluminum. Being lightweight and offering excellent strength and corrosion resistance under high temperatures, titanium metal is used in a wide range of industrial applications.
The examples include jet engine material, structural members and other parts used in aircraft and spaceship, materials for heat-exchangers used in thermal or nuclear power generation, catalyst materials for use in polymer chemistry, and articles of daily use such as eyeglass frame and golf club head. Titanium is also used in various other fields including health products, medical equipment and dental materials, and the applications of titanium are expected to grow further. Titanium is already competing with stainless steel and duralumin, and is likely to become a material that will be in greater demand than these rival metals.
Titanium metal has poor processability and cutting property, and therefore using dispersed titanium material in a production of mechanical parts having a complex shape will require additional machining steps such as cutting after hot forging, rolling or other plastic working process. This inevitably increases process steps and adds to production costs.
For the above reason, powder metallurgy is often used in applications in which titanium metal is used, as mentioned earlier, and accordingly there is demand for titanium powder that offers high purity and uniform particle shape and size. When titanium powder is produced using the conventional powder production methods applicable to general metals, the produced titanium powder will pose problems, just like other metals produced in the same manners, in terms of irregularities in particle shape and size, poor economy, and so on. Therefore, development of a method for producing titanium powder that can yield higher purity and more uniform particle shape and size is eagerly awaited.
For example, improved production methods of titanium metal powder using the hydrogenation and dehydrogenation method and rotary electrode method have already found commercial use. The hydrogenation and dehydrogenation method is a technique to heat titanium sponge, dispersed titanium material or titanium scraps generated from cutting/machining processes in a hydrogen atmosphere to cause the titanium material to absorb hydrogen gas, and then crush the embrittled titanium material, after which the crushed titanium is again heated in a vacuum atmosphere to release hydrogen gas and consequently obtain titanium powder. The rotary electrode method uses a round bar formed from dispersed titanium material or processed dispersed titanium material, which is a forged, rolled or otherwise processed version of dispersed titanium, material. This round bar material is rotated at high speeds in an atmosphere of inactive gas such as argon or helium while its tip is dispersed using a heat source such as an arc or plasma arc, and the dripping molten metal is spattered by centrifugal force to obtain spherical powder particles. In the rotary electrode method, controlling the dispersion amount of the dispersing metal is very difficult.
Titanium powder obtained by the hydrogenation and dehydrogenation method has irregular sphericity. Therefore, although such powder can be used for die molding, the heating process must be repeated twice. While a mechanical crushing process using a ball mill may be devised, it will inevitably cause oxygen contamination of the titanium powder. On the other hand the rotary electrode method, wherein molten titanium material is shaped into powder form in an inactive gas atmosphere, produces spherical powder particles with good fluidity and there is no risk of oxygen contamination. However, this method has a drawback of poor solidification of molding material. In addition, both methods use batch processing and therefore the powder production costs are higher.
The atomization method was developed as a titanium powder production method aiming to resolve the aforementioned problems relating to quality and production costs. Under the atomization method, metal material is dispersed in a water-cooled copper crucible using a plasma arc or other heat source and the molten metal is caused to drip from one end of the crucible. Then, an inactive gas such as argon or helium is injected to this molten metal drips to atomize the molten metal and obtain powder. However, this method couldn't achieve significantly lower production costs compared with the conventional methods, since it also used dispersed titanium material and processed dispersed titanium material.
The invention described in Japanese Patent Application Laid-open No. 5-93213 presents a method for producing titanium powder that offers improved sphericity and fluidity to facilitate molding, in a manner requiring less production costs and avoiding oxygen contamination. However, this method, wherein titanium sponge is isostatically pressed in cold state and the solidified bar material is melted in an inactive gas atmosphere, and then an inactive gas such as argon or helium is injected to atomize the molten metal to obtain powder, still didn't provide powder of desired levels of purity as well as uniform sphericity and particle size and the production costs were not ideal, either.